1. Field of the Invention
The present invention relates to an image writing apparatus which writes an image into a record display medium in which a display layer for displaying an image is stacked between a pair of substrates on both of which electrodes are formed, and to the record display medium thus used.
2. Description of the Related Art
In recent years, record display media have come to attract great attentions as media to transfer information other than paper media and electronic display devices. The record display media each use a display element having a capability of memorization. In each record display medium, a display layer and the like for displaying information are stacked between a pair of substrates on both of which electrodes are formed. The record display medium has the functions: an information display function capable of displaying digital information and image information; a memory function capable of maintaining displayed information even in a powerless state; and a rewriting function capable of erasing written information to reuse the medium. This record display medium is also physically characterized in its flexibility and light weight, and has a feature that the medium can be separated from a writing apparatus for writing information after information is written by the writing apparatus. The medium can then be carried about like a paper medium, and the written information can be read with the medium held in hands.
This kind of display medium is called “electronic paper”, for example. Applications of these media to electronic news papers, electronic books, distributions of resident cards in the field of administrative services, etc. have been studied.
Discussions are now made on use of memorable liquid crystal for a display element with capability of memorization which forms part of the “electronic paper”, for example, an electrophoretic element, electrophoresis element, microcapsule, or cholesteric liquid crystal.
FIGS. 1A to 1D show types of record display media classified depending on electrodes.
FIG. 1A shows a matrix type. Plural striped electrodes 3 are formed on each of inner surfaces of substrates 2 opposed to each other with a display layer 1 interposed between the substrates. Those plural electrodes 3 that are opposed to each other with the display layer 1 interposed are arranged to be perpendicular to each other.
FIG. 1B shows a single electrode type. One electrode 4 is formed on the inner surface of one of substrates 2 opposed to each other with a display layer 1 interposed between the substrates 2.
FIG. 1C shows an electrodeless type. A display layer 1 is sandwiched between substrates 2. No electrode is provided.
FIG. 1D shows an optical writing type. A full-plane electrode 4 is formed on each of inner surfaces of substrates which are opposed to each other with a display layer 1 and optical switching layer 5 interposed between the substrates.
An example of use of a record display medium according to the matrix type will be a device as follows. Electrodes arrayed on a two-dimensional matrix composed of X lines and Y rows are applied with a predetermined voltage at a timing. The state of each of pixels respectively positioning at intersections of the two-dimensional matrix is thus controlled to turn ON or OFF to write information (see Patent Document 1.)
Record display media of this type are generally used for liquid crystal display devices. A record display medium of this type requires electrodes arrayed on a two-dimensional matrix corresponding to the number of pixels. Accordingly, at least 1,120 electrodes are necessary even in case of monochrome display by a VGA (Video Graphics Array) panel having 640×480 pixels, for example.
To reduce the number of arrayed electrodes, a technique has been considered, that is, the technique in which the record display medium is provided with receiving means for receiving information as data to be displayed on the record display medium, and display means for converting the received data into driving data to be displayed on the record display medium.
An example which uses a record display medium of the single electrode type will be a device of head-writing type. In this device, a record display medium is conveyed in a direction perpendicular to a one-dimensional head that applies electric or thermal energy to the record display medium to write information (see Patent Document 2.)
In the device of head-writing type, an electric head opposed to one single electrode formed on a record display medium with a display layer interposed therebetween constitutes another electrode opposed to the one single electrode formed on the record display medium. Information is recorded on the display layer by an electric field or current generated between the electric head and the one single electrode of the record display medium, or by Joule heat generated by the current.
In a record display medium using electrophoresis or in a record display medium using a display element micro-encapsulated liquid crystal or coated liquid crystal contained in a binder, information is recorded by applying a voltage between the electric head and the one single electrode of the record display medium. As an example using a record display medium of the electrodeless type will be a device of head-writing type using a thermal head (see Patent Document 3.)
The thermal head controls the amount of heat to be applied to the record display medium by controlling a current to be flowed through a heat generation member provided in the head. A display layer in a record display medium of the electrodeless type is formed of coloring material such as a leuco dye for example, which is heat-reversible. Controlled heat which expresses information is applied to the display layer from the thermal head. The information is then displayed in colors.
An example using a record display medium of the optical writing type will be an elgraphy technique (see Non-Patent Document 1 and Non-Patent Document 2) or a rewritable printer (see Patent Document 4.)
The record display medium of the optical writing type has a structure in which a display element and an optical switching element are sandwiched between a pair of electrodes. An optical image writing apparatus has a voltage application section and an exposure section. A driving voltage is applied to the pair of electrodes from the voltage application section, and light which transfers information is irradiated on the optical switching element from the exposure section. An optical image is thus written into the display element. In that area of the record display medium that is irradiated with light, resistance of the optical switching element decreases and a high voltage is applied to the display element in this area, and accordingly information is written. In the other remaining area that is not irradiated with light, resistance of the optical switching element does not change but is maintained and no high voltage is applied, and accordingly information is not written.
In addition to the record display media described above, there is another record display medium of the matrix type. This medium has a power source circuit including a power supply coil, a drive circuit, and a data transmitter/receiver circuit. In this type, electric power is supplied by electro-magnetic induction, and data expressing information to be displayed on a display element is transmitted/received wirelessly.
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2000-347225 (paragraphs 0020 to 0022 and FIGS. 2 to 5)
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 04-073615 (embodiments and FIGS. 1, 3, and 5)
Patent Document 3: Japanese Patent Application Laid-Open Publication No. 10-52973 (paragraph 0046 and FIG. 3)
Patent Document 4: Japanese Patent Application Laid-Open Publication No. 4-9964 (embodiments and FIGS. 1 to 3)
Non-Patent Document 1: SID 01 DIGEST (pp 362 to 365)
Non-Patent Document 2: JAPAN Hardcopy '96 Fall Meeting (pp 25 to 28)
Record display media as described above, however, result in the following problems in writing into display elements.
That is, users may intend to write information into a large number of record display media, to separate the media from a writing apparatus, and to carry the media with themselves. However, no writing means has been realized with high reliability and with ability to write information rapidly into a large number of record display media and to write different information items into one same record display medium respectively for many times.
As disclosed in Patent Document 1, plural record display media of the matrix type may be connected to a connector, and sequential or simultaneous writing may be carried out from a writing apparatus. According to this technique, writing into all the connected record display media can be completed in a very short time if the media can be connected to the connector in a very short time. However, a mechanism which attaches the connector to the writing apparatus is complicated. Also, as one same record display medium is used repeatedly and the medium is connected to the connector very often, it is therefore difficult to maintain reliability about the contact points of the connector.
In a writing apparatus of the head writing type using electrodeless record display media, information is written while conveying each record display medium. Therefore, information can be written in a short time by improving the conveyor means for conveying the record display media so as to achieve a higher speed. Meanwhile, a thermal head which thermally writes information has a simpler structure than the means which writes information into record display media of the matrix type, and also, manufacturing costs for the thermal head can therefore be lowered. However, the thermal head directly contacts each record display medium. Therefore, dust and the like which have stuck to a record display medium are transferred to the thermal head while the record display medium is used repeatedly. The dust may be miswritten as a stripe on the record display medium and cause a problem of defectiveness in writing. It is difficult in principle to interpose a protect film or sheet having thickness enough to protect the display element, between the thermal head and the surface of the record display medium that contacts the head. The display element of the record display medium may consequently be damaged due to the dust or foreign material which sticks to the thermal head.
In general, a writing apparatus of the head writing type which uses record display media of the single electrode type applies a voltage between the head and the electrode of the record display medium, to record information on the display element. This apparatus causes the same problem as the aforementioned writing apparatus of the head writing type.
In contrast, record display media of the optical writing type can more steadily ensure resolution regardless of the number of electrodes, compared with the media of the matrix type. Besides, the paired electrodes of the medium of the optical writing type can easily guarantee the reliability in writing information. Further, the writing means does not contact the record display media, so that dirt of any record display medium is not transferred to the writing apparatus and does not further affect another record display medium. Reliability can thus be attained easily in writing information in this type of medium. Furthermore, a driving function based on data need not be provided in each record display medium because the data are not transferred from the writing means to the record display.
Particularly, a record display medium of the optical writing type, which uses an optical switching element and has been invented by the present inventors, can be manufactured by printing processes, and mass production of which is therefore so easy that advantages are attained in costs.
However, when writing an image into plural record display media of the optical writing type, a problem arises in the technique described in Non-Patent Document 1. This technique describes that an image is written with the record display media set on a display or exposure panel and the media are detached from the writing apparatus after the writing. In this technique however, operation must be repeated for every one of the record display media, resulting in difficulties in achieving a higher speed.
Further, there is also a problem in another technique disclosed in Patent Document 4 describing an information recording section in which an information recording medium and an optical switching layer are stacked. High-speed printing is carried out by a printer which has the information recording section further provided with a means for supplying the optical switching layer with print information and a means for conveying/discharging the medium. This technique adopts a structure in which the information record medium and the optical switching layer are separate, and thus the tolerable number of rewritings depends on lifetime of a photoconductive layer. If dust or dirt sticks to an information recording medium which has been used plural times, unnecessary space is created between the medium and the optical switching layer and causes electric loss. Besides, a proper voltage cannot be applied to the information recording medium due to electrical characteristics of the dust or dirt. As a result, quality of a displayed image deteriorates.
To avoid this problem, a proper space may be prepared but need a means for maintaining the proper space and a driving means of a high voltage. A resulting apparatus may then be not only expensive but also unuseful in practice from the viewpoint of energy saving.